The present invention relates to a vehicle four wheel drive front/rear coupling device control device and to a method of operation thereof, for a vehicle adapted for four wheel drive operation and incorporating a four wheel drive power transmission system, and more particularly relates to such a vehicle four wheel drive front/rear coupling device control device and method of operation thereof for a vehicle such as an automobile incorporating such a four wheel drive power transmission system, said front/rear coupling device control device and method providing control according to the effective discrepancy between the rotational speed of the rear vehicle wheels and the rotational speed of the front vehicle wheels, which is capable of performing a learning process for determining the discrepancy between the effective radius of the front vehicle wheels and the effective radius of the rear vehicle wheels.
The present invention has been described in Japanese Patent Application Ser. No. Showa 61-230544 (1986), which was filed by an applicant the same as the entity assigned or owed duty of assignment of the present patent application; and the present patent application hereby incorporates into itself by reference the text of said Japanese Patent Application and the claims and the drawings thereof; a copy is appended to the present application.
Nowadays a greatly increasing number of automotive vehicles are being constructed with four wheel drive transmission systems, because such four wheel drive operation, in which all four wheels of the vehicle are powered from its engine via its transmission, is very suitable for driving on poor or slippery road surfaces such as in mud or over bad ground, or upon roads covered with mud, snow, ice, or rain. In other words, four wheel drive operation provides a much higher degree of stability and drivability for the vehicle in conditions where the coefficient of friction between the wheels and the surface upon which the vehicle is being operated is relatively low. Also, four wheel drive operation is beneficial for aiding with hill climbing characteristics and high speed stability characteristics. Therefore, the so called full time four wheel drive type of transmission, which remains always engaged to four wheel drive without any episodes of two wheel driving, is becoming more and more popular.
In such a four wheel drive transmission system for an automotive vehicle, it is usual to provide a center differential device for distributing rotational power between the front wheels of the vehicle and the rear wheels of the vehicle, as well as the per se conventional rear differential device that provides differential action between the two rear vehicle wheels and the also per se conventional front differential device that provides differential action between the two front vehicle wheels. Such a central or front - rear differential device is provided in order to provide a differential action between said front vehicle wheels (considered as a pair) and said rear vehicle wheels (also considered as a pair) when the vehicle is turning around a curve, in order to eliminate the possiblity of the occurrence of the so called tight corner braking phenomenon created by the difference in the turning radiuses of the front wheels of the vehicle and the rear wheels thereof (and also for various other reasons). And such provision of such a central or front - rear differential device is effective for achieving this result. Further, it is per se conventional to provide, to such a center differential device, a torque distribution control clutch such as a central differential control clutch, which serves for regulating the distribution of the drive torque produced by the engine of the vehicle between the rear wheels of the vehicle (taken as a combination) and the front wheels of the vehicle (taken as a combination). Such a type of construction is disclosed, for example, in Japanese Patent Application Laying Open Publication Ser. No. 50-147027 (1975), in Japanese Patent Application Laying Open Publication Ser. No. 55-72420 (1980), and in Japanese Patent Application Laying Open Publication Ser. No. 57-15019 (1982). And such a torque distribution control clutch such as a central differential control clutch is typically controlled by a hydraulically operated servo device, so that the engagement pressure of said torque distribution control clutch, i.e. the maximum amount of torque that said torque distribution control clutch can transmit, which defines the amount of torque redistribution which said torque distribution control clutch can provide between the rear wheels of the vehicle (taken as a combination) and the front wheels of the vehicle (taken as a combination), is regulated by the magnitude of an actuating hydraulic fluid pressure. And such an actuating hydraulic fluid pressure is typically provided by a control system such as a hybrid electrical/hydraulic control system which may include a microcomputer.
Such a torque distribution control clutch or central differential action restriction means is typically provided for the following reason. If even one of the vehicle wheels slips, which may well occur especially when the vehicle is being operated upon a bad road surface such as when it is raining, it is snowing, or when the road is muddy, drive power will be lost, whereupon there is the problem that because of the differential effect of the center differential device the drive power of all wheels will be reduced, and the so called trailblazability of the vehicle will be severely deteriorated. In, therefore, a four wheel drive device having a center differential device, such a differential restriction device is typically provided, and is typically operated according to vehicle operational conditions. A four wheel drive device has already been proposed, constructed so that, when the difference between the rear wheel revolution rate and the front wheel revolution rate is at least a certain value, that is, when one vehicle tire is slipping with respect to the road surface, the differential control clutch is engaged, and the rear wheels and front wheels are directly coupled, whereas at other times the differential control clutch is released and the center differential device is allowed to carry out a differential effect; this is described in the above mentioned Japanese Patent Laying Open Publication No. Sho 55-72420 (1980). With the provision of such a differential control clutch, when said clutch is engaged then a condition of coupled or substantially coupled front/rear wheel operation of the vehicle is set up. This means that the driving characteristics of the vehicle are improved, so that, for example, when the vehicle is being operated upon a bad road surface such as when it is raining, it is snowing, or when the road is muddy, the vehicle is less likely to skid or get out of control, and further any single one of the vehicle wheels which would otherwise slip, for example if said when should fall into a wet hole or the like, is prevented from thereby suffering a loss of drive power.
On the other hand, there is a problem that arises with regard to the operation of such a front/rear coupling device for a four wheel drive vehicle like the above described type of clutch for such a central differential device. Namely, since the condition of at least one of the vehicle wheels being the slipping condition is determined according to whether or not the discrepancy (difference or ratio) between the rear wheel revolution rate and the front wheel revolution rate is, or is not, at least a certain value, thereby the setting of this value requires consideration of the fact that in actual operation of the vehicle, although the sizes (diameters) of the front vehicle wheels and of the rear vehicle wheels and of the tires mounted on them are nominally equal, nevertheless in practice, due to differences in weight distribution of the vehicle and due to differences in tire inflation pressures and tire wear amounts, the effective rolling radiuses of the front vehicle wheels and of the rear vehicle wheels may differ. This problem is exacerbated because this difference between the effective rolling radiuses of the front vehicle wheels and of the rear vehicle wheels may vary dynamically, not only over a period of months during the use of the vehicle but even during a single operational episode of said vehicle--as the tires fitted to the vehicle wheels become heated up through use, for example.
If this matter of the difference between the effective rolling radiuses of the front vehicle wheels and of the rear vehicle wheels is ignored completely during the operation of the front/rear coupling device like the above described type of clutch for such a central differential device, and merely said front/rear coupling device is engaged according to whether or not the discrepancy (difference or ratio) between the rear wheel revolution rate and the front wheel revolution rate is, or is not, at least a certain threshold value, then the following problems will arise. If this threshold value is a relatively low value, then there is a risk of engaging the front/rear coupling device during normal vehicle operation when none of the vehicle wheels is slipping, and this will partially or completely negate the initial purpose for providing the central differential device. Not only are abnormally high tire wear and fuel consumption likely to eventuate, but also abnormally high loads will be imposed upon various components in the vehicle power train, and this will likely deteriorate the reliability and the service life of said vehicle power train. On the other hand, if the threshold value is a relatively high value, then there is a risk of not engaging the front/rear coupling device during abnormal vehicle operation when one or more of the vehicle wheels is or are slipping, and this will partially or completely negate the initial purpose for providing the front/rear coupling device to the central differential device. This is likewise unsatisfactory; and, further, when the front/rear coupling device does finally engage due to increase of the discrepancy between the rear wheel revolution rate and the front wheel revolution rate above even this relatively high threshold value, then as a matter of course an abnormally high load will be imposed upon said front/rear coupling device, and this will likely deteriorate the reliability and the service life of said front/rear coupling device.